The overall objective of the proposed research is to develop further techniques using the radio isotope 23Mg and two stable isotopes, 25Mg and 25Mg, as in vivo tracers for the study of magnesium utilization in animals and man. Two goals of the proposed research are measurement of magnesium availability from human foods and determination of magnesium status in human subjects. The s research will include experiments with human subjects in free living conditions, obese patients undergoing weight reduction and non-obese studies in a metabolic ward, and rats as test animals. The isotopes will be used as double tracers 1) to measure true absorption of an extrinsic or intrinsic food label using 28Mg i.v. and 26 Mg orally, 2) to compare extrinsic and intrinsic food labels using two stable isotopes, 25Mg and 26Mg, and rats, and 3) to compare absorption and turnover rates in patients following magnesium deletion consequent to weight reduction and after repletion with magnesium supplements. Analysis of 26Mg will be initially by neutron activation analysis. Subsequently, measurement of both 25Mg and 26Mg will be done by high resolution mass spectrometry of a volatile magnesium chelate. The long range goal of the research is to establish stable isotope techniques for measurement of Mg availability and utilization in human subjects.